The 3G-H.324M standard advanced by the International Telecommunication Union (ITU) is an example of a protocol for videoconferencing in a mobile wireless communication network. The 3G-H.324M standard specifies how mobile terminals should handle streaming of audio and video data. The ITU H.245 standard specifies a signaling channel for video call setup. Using H.245 control messages, mobile terminals exchange capabilities and open video, audio and data channels with one another. One or more CODECs handles encoding and decoding of video and audio data for transmission between the mobile terminals to support videoconferencing. The ITU H.223 layer multiplexes and demultiplexes the signaling and data channels.
During H.324M call setup, both mobile terminals indicate their capabilities by exchanging numbered simple retransmit protocol (NSRP) packets containing H.245 call control messages. In general, NSRP is a “stop and wait” protocol in which a terminal sends one packet, waits for acknowledgement from a remote terminal and, only after receiving the acknowledgement, proceeds with transmission of the next NSRP packet. For example, upon receiving an incoming NSRP packet from a sending terminal, a remote terminal will send out an NSRP acknowledgement (ACK) packet to acknowledge to the sending terminal that it indeed received the NSRP packet.
Upon receipt of an NSRP ACK packet, the sending terminal can send out the next NSRP packet and continue with H.324M call setup. If the sending terminal does not receive an NSRP ACK packet within an NSRP timeout period, however, the sending terminal retransmits the previous NSRP packet to the remote terminal. The NSRP ACK packet ensures that H.245 signaling messages are received by the remote terminal, providing guaranteed delivery. However, the “stop and wait” aspect of NSRP can cause undesirable delays during call setup, particularly when the underlying wireless network is error prone and causes a significant number of NSRP packets to be dropped.